1. Field of the Invention
The present invention concerns the fabrication of electrolytic capacitors with solid, organic electrolyte and, more particularly, those where the base metal is aluminium.
2. Description of the Prior Art
Electrolytic capacitors are used especially because of their high capacitance in a restricted volume. At present, there are three classes of electrolytic capacitors in the market. There are aluminium and liquid electrolyte capacitors where the anode is made of aluminium and the cathode is an electrolytic liquid. There are also tantalum capacitors with solid electrolyte, where the anode is made of tantalum and the cathode is a solid semiconducting electrolyte. Finally, there are aluminium capacitors with solid electrolyte: the anode is made of aluminium and the cathode is a solid semiconducting electrolyte.
The last-named class of electrolytic capacitors has undoubtedly seen development recently. The firm Philips has developed several ranges of aluminium electrolytic capacitors with solid electrolyte. The anode is made with a rolled or folded foil. The manufacturing method using a anode in the form of foil comprises the following steps:
punching the aluminium foil,
etching the foil,
folding the etched aluminium foil in the case of radial capacitors or rolling it up in the case of axial capacitors,
anodization of the foil to form a thin film of alumina,
formation of the solid electrolyte (manganese dioxide) by impregnation of a manganese nitrate solution II followed by a pyrolysis operation;
positioning of a cathode contact.
The method used to manufacture these capacitors is fairly complicated. It comprises a one-by-one folding operation for radial capacitors or a one-by-one rolling operation for axial capacitors. The solid electrolyte forming step is the most difficult one. Several cycles of pyrolysis (four in principle) are required, and these cycles must take place under well-determined conditions relating to temperature and time. These are very difficult operations. For, the base product is a highly aggressive solution of manganese nitrate. The conversion into manganese dioxide has to be very fast. A post-formation operation is needed to repair the layer damaged by the nitrogen dioxide resulting from the pyrolysis.
There is a method, known through the French patent FR No. 2 583 216, for the manufacture of electrolytic capacitors comprising a step for the winding of anode and cathode foils and and an electrolyte support on a large-diameter wheel. This method has the advantage of simplicity, obtained by the winding method. It can be used to obtain SMC (surface-mounted component) type components. The solid electrolyte used is preferably manganese oxide but the possibility of using an organic electrolyte is also mentioned. For example, it is possible to use 7, 7, 8, 8-tetracyanoquinodimethane (more commonly called TCNQ) salts. These salts are theoretically very valuable, but their use as electrolytes for capacitors raises many problems in implementation.
To overcome this drawback, the invention proposes a novel method which enables the impregnation of the oxidated anodes of capacitors by a TCNQ salt. This method is implemented by electrolytic means.